Soybeans are a major cash crop and investment commodity in North America and elsewhere. Soybean oil is one of the most widely used edible oils, and soybeans are used worldwide both in animal feed and in human food production.
The soybean cyst nematode, Heterodera glycines Ichinohe, was first identified on soybeans in the United States in 1954 at Castle Hayne, N.C. Winstead, et al., Plant Dis. Rep. 39:9-11,1955. Since its discovery the soybean cyst nematode ("SCN") has been recognized as one of the most destructive pests in soybean. It has been reported in nearly all states in which soybeans are grown, and it causes major production problems in several states, being particularly destructive in the midwestern states. See generally: Caldwell, et al., Agron. J. 52:635-636,1960; Rao-Arelli and Anand, Crop. Sci. 28:650-652, 1988; Baltazar and Mansur, Soybean Genet. Newsl. 19:120-122, 1992; Concibido, et al., Crop. Sci., 1993. For example, susceptible soybean cultivars had 5.7-35.8% lower seed yields than did resistant cultivars on SCN race-3 infested sites in Iowa. Niblack and Norton, Plant Dis. 76:943-948, 1992.
Although the use of nematocides is effective in reducing the population level of the nematode, nematocide use is both uneconomical and potentially environmentally unsound as a control measure in soybean production. Neither is crop rotation a practical means of nematode control, since rotation with a nonsusceptible crop for at least two years is necessary for reducing soybean losses. Therefore, it has long been felt by soybean breeders, that use of resistant varieties is the most practical control measure.
Screening of soybean germplasm for resistance to SCN was begun soon after the discovery of the nematode in the United States, and Golden, et al. (Plant Dis. Rep. 54:544-546, 1970) have described the determination of SCN races. Although SCN was discovered in North America about 40 years ago, soybean breeding for resistance to SCN has mostly utilized genes from two plant introductions--Peking and PI88788, and while these lines have resistance genes for several SCN races, including race-3, they do not provide resistance to all known races.
The plant introduction PI437654 is the only known soybean to have resistance to SCN races-3 (Anand, Plant Dis. 68:593-595, 1984), 1, 2, 5, 14 (Anand, in Lambertin and Taylor (Eds), pp. 269-276, 1985), 6, and 9 (Rao-Arelli et al., Plant Dis. 76:894-896, 1992). However, PI437654 has black seed coat, poor standability, seed shattering, and low yield, necessitating the introgression of its SCN resistance into elite germplasm with a minimum of linkage drag. Conventional breeding with PI437654 produced the variety "Hartwig" (Anand, Crop Sci. 31:856, 1991), which is more adapted to cultivation and can be used as a source of SCN resistance in future soybean breeding.
Resistance to SCN is multigenic and quantitative in soybean (Mansur et al., Crop Sci. 1993), although complete resistance can be scored qualitatively. Myers and Anand (Euphytica 55:197-201, 1991) estimated that PI437654 has three genes required for complete resistance to SCN race-3, four genes for race-5, and three genes for race-14. The multiple genes and SCN races involved contribute to the difficulty breeders have in developing SCN resistant soybean varieties.
Breeding programs for SCN resistance rely primarily on field evaluations where natural nematode populations occur. However, these populations can be mixtures of undetermined races (Young, Crop Sci. 22:385-388, 1982) and the environment can vary, thereby affecting the overwintering and infection capability of the nematodes (Niblack and Norton, Plant Dis. 76:943-948, 1992). Although evaluations using inbred nematode populations in controlled greenhouse environments are superior, they are prohibitively expensive, and the nematodes are difficult to manage for large breeding programs. These deficiencies in each evaluation method make SCN resistance a difficult trait for soybean improvement.
Genetic markers closely linked to important genes may be used to indirectly select for favorable alleles more efficiently than direct phenotypic selection (Lande and Thompson, Genetics 124:543-546, 1990). The i allele at the I locus, responsible for black or imperfect black seed-coat type, is closely linked in coupling to the SCN resistance allele, Rhg.sub.4 in the variety Peking (Matson and Williams, Crop Sci. 5:477, 1965). The I locus was mapped to linkage group VII of the classical genetic map (Weiss, Crop. Sci. 10:627-629, 1970) and to linkage group A of the public RFLP map (Keim et al., Proc. 4th World Soy. Res. Conf., 1989). In addition, other SCN race-3 resistance loci have been associated with RFLP markers and tentatively mapped to linkage group B or G (Boutin et al., Soybean Genet. Newsl. 19:120-122, 1992) and possibly K (Concibido et al., Crop Sci., in press) using the germplasm source PI209332.
Therefore, it is of particular importance, both to the soybean breeder and to farmers who grow and sell soybeans as a cash crop, to identify, through genetic mapping, the quantitative trait loci (QTL) for resistance to the various SCN races. Knowing the QTLs associated with resistance to the SCN races, soybean breeders will be better able to breed SCN resistant soybeans which also possess the other genotypic and phenotypic characteristics required for commercial soybean lines.